These hipposandals must have been tied to the horse’s hooves in some way.
Probably leather. One countermeasure against cavalry was to spread broken pottery to prevent charges. I’d never heard of these hipposandals before, but my uneducated guess is that these were kept around in answer to that practice. In the 11th c, in Sicily, Hardraada was working for the Byzanitine emperor, and was faced with such a situation before a fight with the saracens. He cut down palm fronds, bound them around the hooves and lower legs of his horse, and ordered his men to do the same. They made a successful charge and slaughtered their enemies.